Remove support for Django 1.7

Author: Surgo

README.md

@@ -8,7 +8,7 @@ node in the cluster using
 ## Requirements
 
 * pymemcache
-* Django 1.7+.
+* Django >= 1.8
 
 It was written and tested on Python 2.7 and 3.5.
 

setup.py

@@ -15,7 +15,7 @@
     license='MIT',
     keywords='elasticache amazon cache pymemcache memcached aws',
     packages=['django_elastipymemcache'],
-    install_requires=['pymemcache', 'Django>=1.7'],
+    install_requires=['pymemcache', 'Django>=1.8'],
     classifiers=[
         'Development Status :: 4 - Beta',
         'Environment :: Web Environment',